Evolving metasomatic agent in the Northern Andean subduction zone, deduced from magma composition of the long-lived Pichincha volcanic complex (Ecuador)

Geochemical studies of long-lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism. The Pichincha Volcanic Complex (Northern Andean Volcanic Zone) consists of: (1) an old, highly eroded edifice, the Rucu Pichincha, whose lavas are mostly andesites, erupted from 1,100 to 150 ka; and (2) a younger, essentially dacitic, Guagua Pichincha composite edifice, with three main construction phases (Basal Guagua Pichincha, Toaza, and Cristal) which developed over the last 60 ka. This structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios, as well as by a sharp decrease of fluid-mobile to fluid-immobile element ratios. Geochemical data indicate that fractional crystallization of an amphibole-rich cumulate may account for the evolution from the Guagua Pichincha andesites to dacites. However, in order to explain the transition between the Rucu Pichincha andesites and Guagua Pichincha dacites, the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic, trace-element depleted, mantle-derived end-member, and a siliceous, trace-element enriched, adakitic end-member. The systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the Rucu Pichincha magmas came from a hydrous-fluid metasomatized mantle wedge, whereas Guagua Pichincha magmas are related to partial melting of a siliceous-melt metasomatized mantle. This temporal evolution implies a change from dehydration to partial melting of the slab, which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted Carnegie Ridge at the subduction system. This work emphasizes the importance of studying arc-magma systems over long periods of time (of at least 1 million of years), in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism.     

Photo-interprétation à différentes échelles et relevés de terrain: exemples de corrélations structurales dans le Bassin Méditerranéen et sur le socle Ouest africain

Résumé L'interprétation structurale des linéaments, décelés sur clichés photographiques pris par satellite, est discutée sur les bases suivantes: d'une part en pratiquant au minimum un changement d'échelle (plus grande); d'autre part en utilisant les relevés de terrain. Des exemples pris dans le Bassin Méditerranéen sédimentaire et sur le socle cristallophyllien de l'Afrique de l'Ouest permettent de définir une méthodologie commune. Ainsi en région sédimentaire, partant de prises de vue par exemple au 1/2 500 000, les échelles au 1/15 000 et au 1/50 000 se révèlent être des compléments précieux en photo-interprétation. De la même façon en zone de socle, les échelles au 1/20 000 et au 1/50 000 sont complémentaires du 1/200 000. De plus grâce aux donnés de terrain, il est possible de qualifier l'information photogéologique et d'en exprimer sa nature. Enfin les problèmes de corrélations entre rhegmatisme de socle et fracturation de couverture sont abordés de façon préliminaire: la similitude directionnelle observée devrait orienter les futures recherches en ce domaine.

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The structural interpretation of disclosed lineaments on satellite's pictures ist discused on the following basis: first, with a minimum scale change (making it bigger); on the other hand using field plotting. The examples took from the Mediterranean sedimentary basin and from the western Africa crystalographic shield allow us to define a common methodology. Thus, in sedimentary area, through 1/2 500 000 scale aerial photographys, the 1/15 000 and 1/50 000 scales would be real valuable complements in photo-interpretation. Same way as, in shield zone, the 1/20 000 and 1/50 000 scales would be complements of the 1/200 000 one. More than that, because of field plotting utilisation is possible to quality photogeological information and even to explain its nature. In short correlation problems are exposed in a preliminary approach; the observed directional similitude could orientate the future research.

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Zusammenfassung Die strukturelle Deutung von Lineamenten, die aus Satellitenphotos abzulesen sind, wird nach folgenden Voraussetzungen erörtert: zum einen wird zumindest eine Vergrößerung des Maßstabes vorgenommen, zum anderen werden Geländebefunde herangezogen. An Hand von Beispielen aus dem sedimentären Mittelmeerbecken sowie dem Kristallo-phyllitischen Sockel Westafrikas ist es möglich, eine gemeinsame Methodik zu definieren. So erweisen sich im sedimentären Gebiet die Maßstäbe 115 000 und 150 000 als wertvolle Ergänzungen zur Photointerpretation zum Beispiel einer Aufnahme von 12 500 000. Entsprechend ergänzen im Bereich des Sockels die Maßstäbe 120 000 und 150 000 die Aufnahme von 1200 000. Darüber hinaus ist es mit Hilfe von Geländedaten möglich, die photogeologische Information zu qualifizieren und zu beinhalten. Schließlich werden die Probleme der Korrelation zwischen Bruchbildung im Sockel und Brücken im Deckgebirge angesprochen: Die beobachtete Annäherung in der Ausrichtung soll zukünftige Untersuchungen auf diesem Gebiet orientieren.

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Petrology of the mafic rocks of the Xigaze ophiolite,Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.     

Neodymium and strontium isotope study of ophiolite and orogenic lherzolite petrogenesis

Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. ε_Nd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a ε_Nd-ε_Sr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high⁸⁷Sr/⁸⁶Sr ratios and as such lie to the right of the correlation line towards seawater compositions.From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago.     

Short‐term forecasting with a multi‐level spectral primitive equation model part I ‐ model formulation

A global baroclinic primitive equation model using the spectral technique has been constructed for short‐ and medium‐range numerical weather prediction. The spectral technique, which is a special case of the Galerkin method, employs spherical harmonic basis functions in the evaluation of all horizontal derivatives. The use of a transform technique allows all the horizontal operations to be performed efficiently and allows physical processes to be evaluated in real space. The model employs a semi‐implicit algorithm for time integration and finite differencing in the vertical. Physical processes include orography, moist convection, large scale precipitation and boundary layer processes.     

Isochron‐burial dating of glaciofluvial deposits: First results from the Swiss Alps

In this study, we use isochron‐burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic ¹⁰Be and ²⁶Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron‐burial age based on an assumed initial ratio and the measured ²⁶Al/¹⁰Be ratio. We suggest that, owing to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20 000 ¹⁰Be atoms/g and <150 000 ²⁶Al atoms/g. Using an ²⁶Al/¹⁰Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron‐burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 and 1.3 Ma. Our results indicate that isochron‐burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations. Copyright © 2017 John Wiley & Sons, Ltd.     

Textural, isotopic and REE variations in spinel peridotite xenoliths, Massif Central, France

Sr and Nd isotope analyses and REE patterns are presented for a suite of well-documented mantle-derived xenoliths from the French Massif Central. The xenoliths include spinel harzburgites, spinel lherzolites and some pyroxenites. They show a wide range of textures from undeformed protogranular material through porphyroclastic to equigranular and recrystallised secondary types. Textural differences are strongly linked to trace element geochemistry and variations in radiogenic isotope ratios. Many undeformed protogranular xenoliths are Type IA LREE-depleted with MORB-type ε_Sr values between − 30.7 and − 23.6, and ε_Nd values + 13.9 to + 9.4. A second group of undeformed xenoliths are Type IB LREE-enriched with higher ε_Sr values (− 22.7 to − 10.6) and lower ε_Nd values (+ 11.9 to + 5.6). Deformed xenoliths with porphyroclastic, equigranular and secondary recrystallised textures are all Type IB (LREE-enriched, ε_Nd < 6.4, ε_Sr > 11.8). It is proposed that two separate events have given rise to the observed mixing arrays: (1) MORB-source depleted mantle was enriched by a component derived from an enriched mantle. Deformation and recrystallisation accompanied this event. (2) Subsequently, unenriched MORB-source mantle interacted with magmas chemically akin to the host basalts, and enrichment occurred with little deformation. Hypotheses of Tertiary mantle diapirism resulting in isochemical deformation and refinement of protogranular mantle to equigranular mantle are untenable because of differences in REE patterns and isotopic ratios between different textural groups.